1. Field of the Invention
The present invention is directed to a phosphor for use in a detector for high-energy radiation, and in particular to such a phosphor having a reduced afterglow (persistence).
2. Description of the Prior Art
A detector for the detection of high-energy radiation can be formed by a phosphor and light detector such as a photodiode or a photomultiplier. Such detectors are widely employed in nuclear medicine and in X-ray diagnostics. The phosphor has the function of absorbing the high-energy radiation and emitting visible light as a consequence of this absorption. This visible light can be detected by a photosensitive element, for example a photodiode, a photomultiplier or a light-sensitive film.
In modern radiation detectors as employed, for example, in X-ray computed tomography, phosphors having extremely low afterglow (persistence) are required in order to achieve an adequately high pulse frequency for the X-ray pulses. A phosphor in widespread used to that purposed is thallium-doped cesium iodide Csl:Tl, however, this phosphor still exhibits an afterglow intensity of approximately 10.sup.-2 through 10.sup.-3 of the initial light intensity 20 msec after the high-energy radiation has been shut off. Phosphors whose afterglow decays significantly faster are required for new-generation radiation detectors.
The oxi-sulfides of the rare earths are more promising phosphors for employment in modern radiation detectors. German OS 36 29 180 discloses a method for the manufacture of a phosphor ceramic with the general composition EQU (Ln.sub.1-x-y M.sub.x Ce.sub.y).sub.2 O.sub.2 S:X
with Ln=Gd, La or Y; M=Eu, Pr or Tb and X=F or Cl with 0&lt;x, y&lt;1. The pigment powder employed as the initial material is thereby filled into a vacuum-tight metal container and is compressed into a ceramic by isostatic hot-pressing. The phosphors obtained in this way, however, exhibit an undesirably pronounced afterglow unless additional measures are undertaken.
In the article "A Scintillator Gd.sub.2 O.sub.2 S:Pr,Ce for X-Ray Computed Tomography," Yamada et al, J.Electrochem. Soc., Vol. 136, No.9, September 1989, pages 2713-2716. The authors propose that a phosphor ceramic of a rare earth oxisulfide be doped with cerium in order to reduce the afterglow. A colored phosphor ceramic whose light yield is reduced is obtained, however, due to the cerium additive. The phosphor is thus degraded in terms of another important characteristic.